Sorry, I keep forgetting that spoilers can be rendered in different ways, based on a user setting. Apparently you forgot, too. ;-)

Anyway, of the five different ways of handling spoiler tags, only one requires the user to click on a link to retrieve the spoiler content. The other four options deliver the content with the page, thus affording no protection in the face of network content scanners.

So for spoilers to help in the way you suggest, all of the following conditions would have to hold:

1. users who wish to protect themselves from NSFW content will have to limit themselves to the "link" spoiler option;
2. writers of NSFW content must enclose that content in spoiler tags;
3. the writers must have a protocol for tagging the spoiler content as NSFW, and follow it.

Frankly, I don't see that happening. (It's possible that un-<spoiler>ed NSFW content could be considered to get the tags added, but that's likely to be regarded as censorship, and rejected out of hand.)

So the user ends up clicking on the link and retrieving the spoiler content anyway. No protection.